Help ACC blows cool not cold air :(
 

Yesterday I had the acc filled up with freon, but it doesn't blow cold air. I can hear the commpressor engage but only cool air comes from the vents. I noticed that the acc radiator fan turns on for a while then turns off and repeates the sequence. Also last year when the acc was working fine it used to condens water around the pipes but now there is no condensed water.What's wrong why am I not getting cold air. The mechanic said that the system has to be drained and cleaned beacause a device that doses (I don't know what is doses freon ?? ) has collected dirt and can't dose properly. Is he right ??